1. Technical Field
The present invention relates to a circuit for matching the load impedance of an electronic device.
2. Description of the Related Art
Electronic circuits operate on a load that is suitably matched to display their nominal performance. This becomes particularly important for radio-frequency circuits, especially for power amplifiers of radio transmitters.
Using impedance matchers is known in the prior art. A solution near to the needs of high-integration radio-frequency systems is known from U.S. Pat. No. 379,494. Said patent discloses an antenna tuner of automatic type that comprises digital control circuits for selecting reactive tuning elements of an antenna impedance coupling network. The tuner system provides rapid transformations of impedance of a multiple antenna to the load resistance to effectively transfer the power of a power amplifier of a portable radio transmitter to each antenna selected above a wide frequency range, for example, from 2 to 80 Mhz. The impedance coupling network comprises series of inductive and capacitive elements in an L-shaped configuration. The inductive and capacitive elements are matchable in an incremental manner in a binary value and are matched automatically by means of a digital control in response to the sensed antenna impedance to switch values of components in a binary sequence. The tuning of the antenna to couple the antenna to the power amplifier is detected by broadband sensors and is continuously monitored by voltage standing wave ratio (VSWR) that initializes and controls the duration of the tuning cycles according to the detected antenna impedance coupling condition.
A similar implementation to the previous implementation and still relating to a circuit with capacitance and inductance to be interposed between the load and electronic circuit and provided with low-loss switches to reconfigure load impedance following load variation is found in the article “Reconfigurable RF Impedance Tuner for Match Control in Broadband Wireless Devices” by Whatley, R. B.; Zhen Zhou; Melde, K. L., Antennas and Propagation, IEEE Transactions on Volume 54, Issue 2, Part 1, February 2006 pages 470-478.
The above implementations are suitable for monolithic integration (if MOS switches are used) or on a module (if MEMS switches are used). Nevertheless, they are affected by different types of problem such as the dimension of the corrector and the cost thereof, frequency sensibility, the complexity of load impedance detection and of control strategy.
The first problem is due to the need to create numerous modulated high impedance inductances, to working frequencies and to the need to conduct great quantities of current if the application is power amplification.
The second problem depends on the correction strategy, which is based on using reactive elements the impedance of which depends on the frequency; wishing to use the corrector for multiband applications will thus require making distinct blocks, to be used according to the application.
The use of LC networks in systems with very broad bands, on the other hand, is impossible.
The third problem depends on the need to know the module and phase of the load impedance exactly, which thus makes complex circuits for monitoring impedance and sophisticated control strategies necessary.